My tandem has two small screws above each bottom bracket, one on each side (so four in total). I've not seen them on any other bike, and I can't figure out what they're for! Suggestions so far have been (a) letting water out (except that they're on the top) and (b) putting oil in (except that I didn't think that was how you're meant to service a bottom bracket - I thought you had to take it apart). I know they're not for an eccentric bracket, as there's other screws elsewhere for that. Picture of screws on rear bracket here: http://s8.postimage.org/z4evs6791/IMG_1950.jpg
In case it's any help, original bike spec page is here: http://www.nkilgariff.com/CBcats/Cat_40/CB15.jpg
All help much appreciated!